Sugar, and especially the high-fructose corn syrup that sweetens many processed foods and nearly all soft drinks, has been justly demonized for adding nutritionally empty calories to our diet and causing metabolic disruptions linked to a variety of diseases. But a closer look at what and how Americans eat suggests that simply focusing on sugar will do little to quell the rising epidemic of obesity. This is a multifaceted problem with deep historical roots, and we are doing too little about many of its causes.

More than a third of U.S. adults and nearly one child in five are now obese, according to the Centers for Disease Control and Prevention. Our failure to curtail this epidemic is certain to exact unprecedented tolls on health and increase the cost of medical care. Effective measures to achieve a turnaround require a clearer understanding of the forces that created the problem and continue to perpetuate it.

The increase in obesity began nearly half a century ago with a rise in calories consumed daily and a decline in meals prepared and eaten at home.

According to the Department of Agriculture, in 1970 the food supply provided 2,086 calories per person per day, on average. By 2010, this amount had risen to 2,534 calories, an increase of more than 20 percent. Consuming an extra 448 calories each day could add nearly 50 pounds to the average adult in a year.

Sugar, it turns out, is a minor player in the rise. More than half of the added calories — 242 a day — have come from fats and oils, and an additional 167 calories from flour and cereal. Sugar accounts for only 35 of the added daily calories.

Demographic changes, and how the food and restaurant industries responded to them, compounded the problem. As more women entered the workforce, family meals and especially home-cooked meals became less frequent. (Relatively few husbands became family cooks, sadly.) From 2005 to 2008, according to the Department of Agriculture, 20 percent of U.S. calories were consumed in fast-food and full-service restaurants, more than triple the amount in 1977-78.

Eating just one meal a week away from home can translate into 2 extra pounds a year for the average person, the department calculated. Although the recent economic downturn forced more people to dine at home, the average adult now eats out nearly five times a week.

Unless calorie counts are reliably listed on the menu, it is impossible to know how many are being consumed in a restaurant — never mind the bread, butter, chips and dip that people eat while waiting to be served.

Portions have grown along with waistlines. Restaurants know that Americans love bargains, and providing huge portions doesn’t add substantially to overhead. Although some weight-conscious diners will share an entree or take home half a meal for another day, most people tend to eat what they are served.

These restaurants are training diners to believe a serving of food is much larger than what dietitians would consider a proper portion. Now, even when eating at home, people pile too much on their plates.

Portions of other foods also have ballooned. Bagels, once 2 ounces and 160 calories, now weigh up to 10 ounces and supply 800 calories even before anything is put on them. A Costco corn muffin has 520 calories, and a healthy-sounding raisin bran muffin from Au Bon Pain has 410. Coffee is no longer a calorie-free beverage. A Starbucks 16-ounce latte with 2 percent milk has 190 calories, and a 16-ounce Frappuccino has 410 — about as much as a 32-ounce Coke, with 416 calories from 26 teaspoons of sugar.

People are often fooled by the healthy aura of a food, like yogurt. Yoplait, which has trumped the market in recent years, is really dessert. Yoplait French Vanilla has 26 grams of sugar, twice as much as a serving of Lucky Charms.

Greek yogurt, the current national passion, is really just strained yogurt, higher in protein, fat (unless fat-free) and calories than regular yogurt. And although frozen yogurt typically supplies 30 calories an ounce, most people consume much more than they realize. The serve-yourself frozen yogurt store that recently opened on my corner provides cups that, if filled, would contain 12 ounces; that’s 360 calories, not counting toppings.

Although restaurants and packaged food producers often say they are simply giving customers what they want, Hank Cardello and colleagues at the Hudson Institute, a policy research organization, recently reported that “among leading consumer packaged goods companies and restaurant chains, those that have grown their better-for-you/lower-calorie foods and beverages over the past five years” achieved better sales growth than their competitors.

Toxic though it may be, the problem is not just this environment of food. Researchers now know that people who struggle with weight are battling evolution itself, which has programmed us to store calories when food is plentiful and, when food is scarce, to reduce calories we expend.

When an overweight person cuts down significantly on what he eats, the body defends itself by using fewer calories. The effect can be long-lasting: If a person’s weight drops to 150 pounds from 250, significantly fewer calories must be consumed daily to stay at that weight than would be necessary if the person had never been overweight.

Even if a 170-pound person loses 20 pounds, he needs 15 percent fewer calories to maintain the new weight than someone who always weighed 150. Short of bariatric surgery, very gradual weight loss — say, no more than 20 pounds a year — may be the only way around this metabolic slowdown. This strategy gives the body and appetite a chance to adjust.

Willpower rarely helps people who struggle with their weight. With constant temptations to eat more, and especially more high-calorie foods, our society must change. Instead of subsidies for corn used to produce high-fructose corn syrup, let the government subsidize more fresh fruits and vegetables. Michael Pollan, an author and journalism professor at the University of California, Berkeley, has said it best: “Eat food. Not too much. Mostly plants.” His newest book, “Cooked: A Natural History of Transformation,” adds a corollary: “And cook it yourself.”

No time? People always have time for what they consider important, and what is more important than your health? Home-cooked food contains better ingredients, and you know what you’re eating.